The present invention relates to a differential capacitive pressure sensor and a fabricating method therefor; and, more particularly, to a differential capacitive pressure sensor that implements a superior linearity and a simple fabricating method therefor.
Various instruments, such as a measuring instrument or a controller, employ a pressure sensor to measure a pressure in a process or a system. The pressure sensor generally uses displacement, deflection, frequency of oscillation, or magnetic-thermal conductivity, each varying in response to a pressure stimulus.
Recently, a semiconductor pressure sensor, having a smaller size and being more combined, is getting attention for the development of a semiconductor technology and a micro-machining technology. Since creep rarely occurs in the semiconductor pressure sensor, a superior linearity can be obtained. Further, the semiconductor pressure sensor is small, lightweight, and very strong against vibration. Compared with a mechanical sensor, the semiconductor pressure sensor is more sensitive, more reliable, and presents a higher production yield.
In a typical semiconductor pressure sensor, a pressure stimulus causes distortion or strain of a diaphragm, which is usually formed of a monocrystalline silicon. Though a natural frequency change of a vibrator or a surface elastic wave occurring on a surface of the diaphragm can be used to convert the distortion or strain thereof into an electrical signal, the typical semiconductor pressure sensor is generally classified into a capacitive type and a piezoresistive type.
The piezoresistive pressure sensor is formed by diffusing impurities onto a semiconductor and has advantages such as easy fabrication and superior linearity. Though a simplified processing circuit can be applied in the piezoresistive pressure sensor, a correction circuit is usually added thereto to overcome a poor temperature characteristic thereof.
In the capacitive pressure sensor, an exterior stimulus, i.e., pressure or stress, causes a change in a gap interposed between opposing electrodes, so that capacitance therebetween is changed. The amount of the changed capacitance is then converted into an electrical signal, which involves with the magnitude of the stress or the pressure. Compared with the piezoresistive type, the capacitive pressure sensor has a smaller size as well as a better temperature characteristic.
The capacitive pressure sensor, however, has a relatively poor linearity, because the capacitance is inversely proportional to the interval between the opposing electrodes. The linearity thereof becomes more rapidly deteriorated as the capacitive pressure sensor is used for measuring a wider range of stimulus.
Mitsuhiro Yamada, et al., have disclosed a compensation method for improving linearity of a capacitive pressure sensor in a paper, xe2x80x9cA Capacitive Pressure Sensor Interface Using Oversampling xcex94xe2x88x92xcexa3 Demodulation Techniques,xe2x80x9d IEEE Transactions on Instrumentation and Measurement, Vol. 46, No. 1, February 1997. Since the above-mentioned method uses a look-up table, data is digitally stored or input to a circuit. Consequently, a continuous compensation is impossible, and therefore an irregular variance in the output of the sensor is a fatal drawback.
For achieving an improved linearity, a differential capacitive pressure sensor is further suggested. When a pressure acts on a typical differential capacitive pressure sensor, a first displacement (+)xcex94d and a second displacement (xe2x88x92)xcex94d are respectively involved with a first sensing capacitor xe2x80x9cC1xe2x80x9d and a second sensing capacitor xe2x80x9cC2xe2x80x9d thereof. The first and the second sensing capacitor xe2x80x9cC1xe2x80x9d and xe2x80x9cC2xe2x80x9d respectively present a first capacitance and a second capacitance, which are also respectively referred to as xe2x80x9cC1xe2x80x9d and xe2x80x9cC2xe2x80x9d for the sake of convenience. Because the absolute value xe2x80x9cxcex94dxe2x80x9d of the first and the second displacement is usually very small, a capacitance difference xe2x80x9cxcex94Cxe2x80x9d between the first and the second capacitance xe2x80x9cC1xe2x80x9d and xe2x80x9cC2xe2x80x9d (xcex94C=C1xe2x88x92C2) is in proportion to the absolute value xe2x80x9cxcex94dxe2x80x9d thereof. Accordingly, the differential capacitive pressure sensor implements a superior linearity, and effects of a parasitic capacitance are almost excluded.
U.S. Pat. No. 5,925,824 by Shinichi Soma, et al., discloses a conventional differential capacitive pressure sensor. In the conventional differential capacitive pressure sensor, an insulator and a conductive plate, each having a concentric through hole, are sequentially assembled on opposing surfaces of a common conductive plate, such that two capacitors are respectively formed on both opposing surfaces of the common conductive plate. The above-explained structure is difficult to fabricate and therefore its production yield is low. Further, the structure is unsuitable for a small-sized sensor and presents a relatively low sensitivity.
ChuanChe Whang, et al., have disclosed another conventional differential capacitive pressure sensor in a paper, xe2x80x9cContamination-Insensitive Differential Capacitive Pressure Sensorsxe2x80x9d, Journal of MEMS, Vol. 9, No. 4, December 2000. The above-mentioned pressure sensor is fabricated by applying a micro-machining technology, so that two sensing capacitors are formed on a membrane that can elastically deflects in response to a pressure stimulus.
The above-mentioned differential capacitive pressure sensor includes a lower electrode, a center electrode, and an upper electrode. The lower electrode is a polysilicon membrane, and the center electrode is supported by a leg formed on a bulk silicon substrate, which presents no deflection. The upper electrode is disposed over the lower electrode and a supporter is interposed therebetween to support the upper electrode. When the lower electrode deflects in response to the pressure stimulus, the upper electrode also deflects as much as the lower electrode does.
Since the above-explained differential capacitive pressure sensor is hermetically sealed, contamination by particles is prevented. Further, because two capacitors thereof are respectively arranged on an upside and a downside, a large fill factor can be obtained, so that the temperature characteristic and the linearity thereof are superior.
In the above-explained differential capacitive pressure sensor, however, because two sacrificial layers are used during a fabrication process therefor, the fabrication process is very complicated. Further, unless gaps between the upper and the lower electrode are continuous, there occurs a difference in capacitances between the two capacitors. Because the thickness of the sacrificial layers determines the above-mentioned gaps, the sacrificial layers are required to have a same thickness. The two sacrificial layers, however, are very difficult to have the same thickness.
Further, because the support is made of an insulating material or a conductive material covered by an insulator, additional processes are required for forming the support or the insulator. Furthermore, because the support interposed between the upper and the lower electrode supports only the upper electrode, the deflection of the lower electrode differs from that of the upper electrode. In other words, the support causes a difference in deflection between the upper and the lower electrode, thereby deteriorating the accuracy of the sensor
It is, therefore, an object of the present invention to provide a differential capacitive pressure sensor that includes two capacitors arranged in such a way that a fabrication process therefor is relatively easy.
It is another object of the present invention to provide a differential capacitive pressure sensor that provides a superior linearity by maintaining an equal gap between an upper electrode and a lower electrode of each capacitor.
In accordance with one aspect of the invention, a preferred embodiment of the present invention provides a differential capacitive pressure sensor, which includes: a substrate including a diaphragm positioned at a center portion thereof, the diaphragm having a reduced thickness; a first to a third lower electrode sequentially arranged on the substrate, the second lower electrode being positioned corresponding to the diaphragm, the first lower electrode and the second lower electrode being electrically connected to each other; a first to a third upper electrode respectively floating over the first to the third lower electrode, the second upper electrode crossing over the second lower electrode; and a first to a third supporting member downwardly extending from the first to the third upper electrode, respectively, the first and the second supporting member contacting the second lower electrode.
Another preferred embodiment of the present invention provides a differential capacitive pressure sensor, which includes: a substrate including a diaphragm positioned at a center portion thereof, the diaphragm having a reduced thickness; a first to a third lower electrode sequentially arranged on the substrate, the second lower electrode being positioned corresponding to the diaphragm, the first lower electrode and the second lower electrode being electrically connected to each other; a first to a third upper electrode respectively floating over the first to the third lower electrode, the second upper electrode crossing over the second lower electrode; and a first to a third supporting member downwardly extending from the first to the third upper electrode, respectively, the first and the second supporting member contacting the second lower electrode.
In accordance with another aspect of the invention, there is provided a method for fabricating a differential capacitive pressure sensor, the method including the steps of: forming lower electrodes on a substrate; forming a sacrificial layer on the substrate to cover the lower electrodes; forming a plurality of via holes passing through the sacrificial layer to uncover portions of the lower electrodes; forming the upper electrodes by depositing a conductive material on the sacrificial layer, the conductive material filling the via holes so that the supporting members are formed; and removing the sacrificial layer.